1. Field of the Invention
The present invention relates to a molding die for molding a boot for a constant velocity joint.
2. Description of Related Art
For example, in vehicles such as an automobile, constant velocity joints are used in order to transmit a rotation force to driving wheels that move upward and downward according to a road surface. A boot for a constant velocity joint made of synthetic resin is fitted to the constant velocity joint, in order to protect the constant velocity joint from external muddy water or dust and to retain lubricating grease supplied to the constant velocity joint. The boot for a constant velocity joint includes a cylindrical larger-diameter portion and a cylindrical smaller-diameter portion, and a bellows portion connecting the larger-diameter portion and the smaller-diameter portion. The boot for a constant velocity joint is generally formed by a blow molding method.
In the above-described blow molding method, molding is performed by setting molten resin formed in the shape of a bag, within an outer die, and by expanding the bag-shaped molten resin using a blow of air so that the molten resin is pressed against the outer die, without using an inner die. Thus, in the blow molding method, an inner peripheral surface of the boot for a constant velocity joint cannot be molded with high precision. Therefore, it cannot be said that the blow molding method is optimal as the method of molding the boot for a constant velocity joint, which requires a precise bellows structure. Thus, as shown in Japanese Patent Application Publication No. 2012-126033 (JP 2012-126033 A), a method of molding a boot for a constant velocity joint by injection molding with use of an outer die and an inner die (core die) is suggested.
The inner die used for the injection molding in JP 2012-126033 A is constituted by a center core formed so that the diameter thereof is gradually increased from an upper end toward a lower end, and a plurality of split dies arranged along an outer periphery of the center core. This inner die is again assembled after the center core and the split dies are separated in order to take out a compact from the outer die when the molding of the boot for a constant velocity joint is completed.
However, in the inner die described in JP 2012-126033 A, a process of pushing up the center core to push the split dies radially outward (the process of changing the state from the state shown in FIG. 3 to the state shown in FIG. 1 in JP 2012-126033 A) is required when the center core and the split dies are fitted to each other. Therefore, when the center core is pushed up, a gap is likely to be formed between the split dies adjacent to each other in the circumferential direction. If injection molding is performed in a state where this gap is formed, burrs are generated in the compact.